1. Field of the Invention
The present invention relates to a method for driving solid-state image pickup devices and an image pickup system and more specifically to a method for driving interlace scanning type solid-state image pickup devices and an image pickup system carrying the solid-state image pickup devices to which such driving method is applied as image pickup devices and controlling an exposure time by using light control means such as a mechanical shutter.
2. Related Art Statement
Digital still cameras (electronic still cameras) are now rapidly disseminating as an image pickup system using solid-state image pickup devices as image pickup devices. The digital still camera uses a so-called total pixel reading out type solid-state image pickup device which reads out signal charges of all pixels simultaneously and transfers the signal charges of the respective pixels independently or a so-called frame reading out type solid-state image pickup device which reads out signal charges of odd and even lines alternately per field and transfers the signal charges of the respective pixels independently is used to give priority to the resolution in taking a still picture.
Signals of two fields are required to obtain one frame of picture when the frame reading out type solid-state image pickup device is used among such reading out type solid-state image pickup devices as the image pickup device. Accordingly, it is required to shade by a mechanical shutter after completing exposure to obtain one picture by one exposure like a digital still camera in order to prevent the images of the first and second fields from changing.
By the way, the charge transfer type solid-state image pickup device typified by the CCD (Charge Coupled Device) image pickup device causes smear and blooming as its unique phenomena. Here, the xe2x80x98smearxe2x80x99 is a phenomenon causing whitish stripe noises across the whole angle of field in the vertical direction when there is a highly luminous subject within an picked up image. The blooming is a phenomenon by which signal charges generated by the sensor sections (pixels) become excessive as an excessive quantity of light enters and overflow to the surrounding pixels, thus causing a white part spreading around the sensor sections.
Beside the smear and blooming components, a semiconductor device causes electric charge stored as time elapses regardless of light even in the state in which light is shut off, i.e., a dark signal (dark current), as a noise component. These noise components appear as stationary pattern noises in the picked up image and become a factor of deteriorating the image quality.
Then, an operation of sweeping out the electric charges within the vertical transfer section by transferring and driving the vertical transfer section at speed higher than the normal transfer speed after closing a mechanical shutter (hereinafter referred to as a sweep-out transfer) has been carried out to sweep the dark signal component sprung out within the vertical transfer section and the smear and blooming components leaked from the sensor sections in the prior art CCD image pickup device used as an image pickup device in a digital still camera controlling an exposure time by using the mechanical shutter. After that, the signal charges from the respective sensor sections are read out.
Although it is possible to obtain one image by one time of exposure without using the mechanical shutter in case of the total pixel reading out type solid-state image pickup device, there is a case when the similar sweep-out transfer is carried out also in the total pixel reading out type solid-state image pickup device to remove the dark signal component sprung out in the vertical transfer section and the smear and blooming components leaked from the sensor sections.
In case of the frame reading out type solid-state image pickup device, the sweep-out transfer is carried out by the same number of steps in the first and the second field sides, respectively, from the point of time when the mechanical shutter is closed and the exposure period ends till the time when the signal charge is read out from the sensor section as it is apparent from a timing chart in FIG. 7. It means that the sweep-out transfer period is equal in the first and the second field sides. That is, it also means that the quantity of handled electric charge in the sweep-out transfer of the first and the second field sides is equal.
However, a quantity of unnecessary electric charge swept out at high speed is different as follows in the first and the second field sides. That is, while the quantity of unnecessary charge swept out at high speed is the dark signal component+smear component+blooming component in the first field side because the smear and blooming components are caused by the input light and are all swept out by the sweep-out transfer in the first field side, only the dark signal component is swept out in the second field side.
Therefore, although there has been totally no problem in the sweep-out transfer in the second field side when an excessive quantity of light enters during the exposure period and when the smear and blooming components increase along that, there has been a problem in the sweep-out transfer in the first field side that the quantity of unnecessary charge to be swept out exceeds the quantity of charge to be handled in the vertical transfer section, thus causing defective sweep.
For example, there is a case of taking a picture of the sun during day time which is otherwise difficult to see directly by eyes because the digital still camera allows its user to take such picture by watching an image projected on a liquid crystal monitor without watching the sun directly by eyes through an optical finder. When such excessive quantity of light like the sun light enters, a quantity of unnecessary charge to be swept out exceeds the quantity of charge to be handled by the vertical transfer section, thus causing the defective sweep which appears as a vertical stripe in the upper part of the screen as shown in FIG. 8.
It is conceivable to take a method of increasing the quantity of handled charge by prolonging the sweep-out transfer period of the first field side to eliminate such problem. Normally, because the dark signal component less than  less than  (smear component+blooming component), it is conceivable of not carrying out the sweep-out transfer in the second field side and of allocating it to the sweep-out transfer period of the first field side when only the smear component+blooming component are to be swept out. In this case, the sweep-out transfer period of the first field side may be doubled without increasing the total sweep-out transfer period.
However, the sweep-out transfer must be carried out also in the second field side due to the following reasons. When the sweep-out transfer period of the second field side is eliminated as shown in the timing chart in FIG. 9A, shading in which a dark signal output waveform (1) in the first field side has an inclination obliquely as shown in FIG. 9B occurs. However, a dark signal output waveform (2) of the second field side becomes constant. Thus, the dark signal output waveforms (1) and (2) are unbalanced in the first and second field sides.
Here, a case of turning on a power supply right after closing the mechanical shutter will be considered to explain it as shown in the timing chart in FIG. 9A. At first, because the sweep-out transfer for removing the smear components and others is fast, there is less storage time. Accordingly, the dark signal component generated during this sweep-out transfer period is fully smaller than the dark signal component generated in a line shift (vertical transfer of one line) period after that, so that it may be considered to be almost zero. This sweep-out transfer sweeps and removes also the dark signal component in addition to the smear and blooming components.
Then, the dark signal increases as the storage time elapses in the vertical transfer section during the next line shift period and presents the dark signal output waveform (1) as shown in FIG. 9B. No dark signal component is removed by the sweep-out transfer before reading out the signal charge from the sensor section and the dark signal A is stored before outputting the signal as shown in FIG. 9B in the next second field side, so that the dark signal output waveform (1)+A, i.e., the dark signal output waveform (2), is outputted in outputting the signal and the dark signal becomes constant.
When the sweep-out transfer is carried out also in the second field side as shown in a timing chart in FIG. 10A on the other hand, shading by which the dark signal output waveform (1) on the first field side has an inclination obliquely occurs and shading by which the dark signal output waveform (2) has an inclination obliquely occurs also in the second field side as shown in FIG. 10B because the dark signal (the dark signal component A which has been stored till then) is removed by the sweep-out transfer before reading out.
Thus, although the dark signal output waveforms (1) and (2) of the first and second field sides have the same waveform as shown in FIG. 10B when the sweep-out transfer is carried out also in the second field side, the shading by which the dark signal output waveform (1) in the first field side has the inclination obliquely occurs and the dark signal output waveform (2) in the second field side becomes constant, thus causing the unbalanced dark signal output waveforms (1) (2) in the first and second field sides as shown in FIG. 9B, when no sweep-out transfer is carried out in the second field side.
When the both fields are combined to create one frame of image, output lines of the first and second fields are disposed alternately. Then, the dark signals differ largely among the lines and the difference of the outputs of the dark signal appears as a horizontal stripe especially around the upper part of the screen (the first step of signal output). Accordingly, even such dark signal component which is fully smaller than the smear and blooming components deteriorates the image quality in such state, so that the sweep-out transfer is actually carried out also in the second field side.
Still more, the sweep-out transfer period of the second field side had to be prolonged accordingly to increase the quantity of handled charge by prolonging the sweep-out transfer period of the first field side as described above because the idea of equally setting the sweep-out transfer periods in the first and second field sides has been set as a premise in the past from such reasons that it is easy to set driving timing. Then, there has been a new problem that the period from the time when the mechanical shutter is closed till when the signal charges of all pixels are outputted is prolonged because the total sweep-out transfer period of the second field is prolonged.
The above and other advantages of the invention will become more apparent in the following description and the accompanying drawings.
The present invention has been devised in view of the problems described above and its object is to provide a method for driving solid-state image pickup devices, and an image pickup system, which allows a quantity of handled electric charge (quantity of swept-out charge) to be increased in the sweep-out transfer period of the first field side without changing the total sweep-out time.
In order to achieve the above-mentioned object, according to a first aspect of the invention, the sweep-out transfer period of the first field side is set to be longer than the sweep-out transfer period of the second field side in a solid-state image pickup device having a plurality of sensor sections which are arrayed in matrix and carry out photoelectric conversion and a vertical transfer section for transferring signal charges photoelectrically converted by those sensor sections and carrying out the sweep-out transfer of transferring and sweeping charges within the vertical transfer section before reading out the signal charges from the sensor section to the vertical transfer section.
According to a second aspect of the invention, transfer speed in the sweep-out transfer period of the second field side is set to be higher than transfer speed in the sweep-out transfer period of the first field side in the solid-state image pickup device.
According to a third aspect of the invention, an image pickup system comprises a solid-state image pickup device having a plurality of sensor sections which are arrayed in matrix and carry out photoelectric conversion and a vertical transfer section for transferring signal charges photoelectrically converted by those sensor sections; wherein the solid-state image pickup device carrying out sweep-out transfer of transferring and sweeping electric charges within the vertical transfer section before reading out the signal charges from the sensor section to the vertical transfer section; light control means for controlling light entering to the respective sensor sections of the solid-state image pickup device; and driving means for driving the solid-state image pickup device and the light control means and setting the sweep-out transfer period of the first field side carried out from when the light control means is closed till when the signal charges are read out from the sensor section to the vertical transfer section to be longer than the sweep-out transfer period of the second field side.
According to a fourth aspect of the invention, transfer speed in the sweep-out transfer period of the second field side is set to be higher than transfer speed in the sweep-out transfer period of the first field side.
According to a fifth aspect of the invention, there is provided a method for driving a solid-state image pickup device having a plurality of sensor sections which are arrayed in matrix and carry out photoelectric conversion and a vertical transfer section for transferring signal charges photoelectrically converted by those sensor sections and carrying out sweep-out transfer of transferring and sweeping charges within the vertical transfer section before reading out the signal charges from the sensor section to the vertical transfer section. The method comprises a first sweep-out transfer step of transferring and sweeping charges within the vertical transfer section after the exposure period of the sensor section; a first charge reading out and transferring step of reading out and transferring the charges in the first field from the sensor section to the vertical transfer section after the first sweep-out transfer step; a second sweep-out transfer step of transferring and sweeping the charges within the vertical transfer section after the first charge reading out and transferring step; and a second charge reading out and transferring step of reading out and transferring from the sensor section to the vertical transfer section after the second sweep and transfer step. In the method, the sweep-out transfer period of the first field side is set to be longer than the sweep-out transfer period of the second field side.
According to a sixth aspect of the invention, the method for driving the solid-state image pickup device further comprises a step of blocking light from entering to the sensor section after the exposure period in the sensor section and before the first sweep-out transfer period.
The sweep-out transfer period of the first field side is set to be longer than that of the second field side in mounting the solid-state image pickup device which carries out the sweep-out transfer before reading out the signal charges from the sensor section to the vertical transfer section in the image pickup system which controls the exposure time by using light control means, e.g., the mechanical shutter. Because the quantity of charge handled in the second field side is smaller than that in the first field side, it is preferable to allocate the period corresponding to the small quantity to the sweep-out transfer period of the first field side. It allows the quantity of electric charge to be handled in the sweep-out period of the first field side to be increased without changing the total sweep-out period.